The present invention relates to a device for trimming and automatic cutting of multiple images or a single image, obtained in particular by digital rendering on paper or other graphic or photographic supports wound in rolls or in the form of single sheets, especially of large size.
It is known from publication EP-A-0951973, in the name of the same applicant, to provide an automatic cutting device which allows to separate one by one multiple copies in a rapid and precise manner according to the positioning as set by the software of the digital rendering system itself in the printing stage by previously inserting between the copies or prints, where the cutting is required, marks of optical reflection, easily detectable by continuous scanning. In this way the angular correction of the cut can be carried out either with respect to a mark, as it was made previously, at the edge of an opaque material being laminated onto a transparent material, or with respect to images printed not at right angles to the substrate. It was thus possible to obtain the cutting on substrates in the form of reels or as sheets, even if free from any guide system.
On the other hand it is known that more recently the market has shown the need of extending, up to doubling its value, the maximum size of the cutting units to be used with paper sheets having a length greater than one meter. In this case, some difficulties are met when introducing the substrate into the cutting unit, in both cases of a continuous reel or of single sheets, if the pair of feeding rollers extends throughout the cutting length. Due to the material flexibility and weight in fact the parallel and simultaneous introduction of the total sheet width cannot be ensured. If an angle of the sheet is introduced with some delay with respect to the remainder of the sheet itself or of the unwinding reel, the substrate would reach the cutting station in an incorrect position and, while trying to recover the error upon control of the optical sensors detecting the same, the feeding rollers would cause unacceptable wrinkling of the substrate itself.
On the other hand, the substrate introduction steps which require excessive care in the alignment would involve too much labor being engaged, thus lengthening the time of production to become anti-economic to the detriment of the advantages which could be obtained with the above-mentioned prior art as described in the said EP publication. Therefore a solution appears to be that of adopting a pair of feed rollers having a reduced length with respect to the cutting length, such as to occupy only the central zone of the support width. In this way, since the feeding rollers have more orientation freedom, there is no problem with an imperfect alignment at the paper introduction, which thereby requires less care and allows a speedier cutting execution. However there is a derived problem, consequent to the fact that, upon detecting the mark through both the sensors, the possible angle of deviation from a correct feed, the substrate with the associated mark is caused to be fed under the control of the microprocessor through a length corresponding to the distance between the reading system and the cutting line. When travelling along said feed length, also called “offset”, the paper substrate must keep the same angle as initially detected by the sensors. However, for reasons of weight and/or friction, combined with the substrate being held only in a restricted central area, it cannot skid from the rollers nip and modify the feed angle with respect to the cutting station, once concluded the offset transportation.